1. Field of the Invention
The present invention relates to an optical information recording/reproduction method and apparatus for recording/reproducing information by irradiating a light beam onto an optical information recording medium and, more particularly, to an optical information recording/reproduction method and apparatus for performing auto-tracking control using a 3-beam method.
2. Related Background Art
As information recording media for recording/reproducing information using light, various types of media such as disk-shaped media, card-shaped media, tape-shaped media, and the like are known. Some of these optical information recording media allow both recording and reproduction and some others allow only reproduction. Information is recorded on a recordable medium by scanning an information track with a light beam which is modulated according to recording information and is focused to a small spot shape. In this case, information is recorded as an optically detectable information bit string.
Information is reproduced from a recording medium by scanning the information bit string on the information track with a light beam spot of a constant power which does not allow recording on the medium, and by detecting light reflected by or transmitted through the medium.
An optical head used for recording/reproducing information on/from a recording medium is movable relative to the recording medium in the information track direction of the medium and a direction intersecting the information track direction. Upon movement of the optical head, a light beam spot is scanned on an information track. As a lens for focusing a light beam spot in the optical head, for example, an objective lens is used. The objective lens is held by an optical head main body to be independently movable in its optical axis direction (focusing direction) and a direction (tracking direction) perpendicular to both the optical axis direction and the information track direction of the recording medium. The objective lens is normally held via an elastic member, and the movement of the objective lens in the two directions is normally attained by an actuator which utilizes a magnetic interaction.
Of the above-mentioned optical information recording media, a card-shaped optical information recording medium (to be referred to as an optical card hereinafter) is compact and light in weight, and a large demand is expected in the future as portable, relatively large-capacity information recording media.
FIG. 1 is a plan view of a write-once type optical card, and FIG. 2 is a partial enlarged view thereof.
Referring to FIG. 1, a large number of parallel information tracks 2 are arranged in an L-F direction on the information recording surface of an optical card 1. Also, a home position 3 serving as a reference position for access to the information tracks 2 is defined on the information recording surface of the optical card 1. The information tracks 2 (2-1, 2--2, 2-3, . . . ) are arranged in turn from a position near the home position 3, and tracking tracks 4 (4-1, 4-2, 4-3, . . . ) are arranged in turn at positions neighboring the corresponding information tracks, as shown in FIG. 2. These tracking tracks 4 are used as guides for auto-tracking (to be abbreviated as AT hereinafter) for controlling a light beam spot from falling outside a predetermined information track upon scanning of the beam spot in an information recording/reproduction mode.
The AT servo is realized as follows. That is, an optical head detects a deviation (AT error) of the light beam spot from the information track, and negatively feeds back the detection signal to a tracking actuator. The tracking actuator moves an objective lens in the tracking direction (D direction) with respect to the optical head main body, thus tracking the light beam spot on a desired information track.
In the information recording/reproduction mode, upon scanning of the information track with the light beam spot, auto-focusing (to be abbreviated as AF hereinafter) servo is executed to form (focus) the light beam into a spot shape having a proper size on the optical card surface. The AF servo is attained as follows. That is, the optical head detects a deviation (AF error) of the light beam spot from the in-focus state, and negatively feeds back the detection signal to a focusing actuator. The focusing actuator moves the objective lens in the focusing direction with respect to the optical head main body, thus focusing the light beam spot on the optical card surface.
Note that S.sub.1, S.sub.2, and S.sub.3 in FIG. 2 represent light beam spots. AT control is performed using the light spots S.sub.1 and S.sub.3, and AF control, formation of information bits in a recording mode, and reading of the information bits in a reproduction mode are performed using the light spot S.sub.2. Each information track has pre-formatted left address portions 6-1 and 6-2 and right address portions 7-1 and 7-2. By reading out the address portion, the information track is identified. On a data portion 5 (corresponding to 5-1 and 5-2 in FIG. 2), predetermined information is recorded.
An optical information recording method will be explained below with reference to the schematic view of an optical head optical system shown in FIG. 3.
Referring to FIG. 3, a semiconductor laser 21 serves as a light source, and emits light having a wavelength of 830 nm, which is polarized in a direction parallel to the plane of the drawing. The optical system also includes a collimator lens 22, a beam shaping prism 23, a diffraction grating 24 for splitting a light beam, and a polarization beam splitter 25. Furthermore, the optical system includes a quarterwave plate 26, an objective lens 27, a cylindrical lens 29, and a photodetector 30.
A light beam emitted from the semiconductor laser 21 is incident on the collimator lens as a divergent light beam. The divergent light beam is converted into a collimated light beam by the collimator lens, and the collimated light beam is shaped by the beam shaping prism 23 into a beam having a predetermined light intensity distribution, i.e., a circular intensity distribution. The light beam is incident on the diffraction grating 24, and is split into three effective light beams (a 0th-order diffracted light beam and .+-.1st-order diffracted light beams). These three light beams are incident on the polarization beam splitter 25 as p-polarized light beams. The polarization beam splitter 25 has spectrum characteristics shown in FIG. 4, and the incident p-polarized light beams are transmitted through the beam splitter 25 by almost 100%.
The three light beams are transmitted through the quarterwave plate 26. In this case, these light beams are converted into circularly polarized light beams, and the circularly polarized light beams are focused on the optical card 1 by the objective lens 27. The focused light beams correspond to three small beam spots S.sub.1 (+1st-order diffracted light), S.sub.2 (0th-order diffracted light), and S.sub.3 (-1st-order diffracted light), as shown in FIG. 2. The spot S.sub.2 is used for recording, reproduction, and AF control, and the spots S.sub.1 and S.sub.3 are used for AT control. The spot positions on the optical card 1 are as shown in FIG. 2. That is, the light beam spots S.sub.1 and S.sub.3 are located on the neighboring tracking tracks 4, and the light beam spot S.sub.2 is located on the information track 2 between these tracking tracks. Reflected light beams from the light beam spots formed on the optical card 1 are converted into collimated beams via the objective lens 27 again, and are then converted via the quarterwave plate 26 into light beams whose direction of polarization is rotated through 90.degree. from that in the incident state. These light beams are incident on the polarization beam splitter 25 as s-polarized light beams, and are reflected by almost 100% due to the spectrum characteristics shown in FIG. 4. The reflected light beams are then guided to a detection optical system.
In the detection optical system, a spherical lens 28 and the cylindrical lens 29 are combined. With this combination of the lenses, AF control based on an astigmatism method is performed. The three beams reflected by the optical card 1 are respectively focused by the detection optical system, and are incident on the photodetector 30 to form three light spots. The photodetector 30 comprises light-receiving elements 30a and 30c, and a four-split light-receiving element 30b, as shown in FIG. 5. The light-receiving elements 30a and 30c receive the reflected light beams of the light spots S.sub.1 and S.sub.3, and the AT control is performed using the difference between the outputs from these two light-receiving elements. Also, the four-split light-receiving element 30b receives the reflected light beam of the light spot S.sub.2, and the AF control and reproduction of recorded information are performed using the output from this light-receiving element.
Processing of the signals detected by the detection optical system will be described below with reference to FIG. 6. The reflected light beams from the light spots S.sub.1, S.sub.2, and S.sub.3 formed on the optical card 1 respectively form light spots S.sub.a, S.sub.b, and S.sub.c on the light-receiving elements 30a, 30b, and 30c of the photodetector 30.
Outputs from split portions in the respective diagonal directions of the four-split light-receiving element 30b are respectively added by adders 117 and 118. The outputs from these adders 117 and 118 are added by an adder 121, thus obtaining an information reproduction signal RF. More specifically, the information reproduction signal RF corresponds to the total light amount of the light spot S.sub.b formed on the four-split light-receiving element 30b. The output from the adder 118 is subtracted from the output from the-adder 117 by a differential circuit 120, thus obtaining an AF control signal Af.
The output from the light-receiving element 30c is subtracted from the output from the light-receiving element 30a by a differential circuit 119, thus obtaining an AT control signal At. Normally, the AT control is performed to make the AT control signal At zero. When the two light spots S.sub.1 and S.sub.3 on the optical card 1 shown in FIG. 2 overlap the corresponding tracking tracks (grooves) 4-2 and 4-3 by equal areas, the light amounts of the light spots S.sub.a and S.sub.c on the light-receiving elements 30a and 30c become equal to each other. Therefore, when the control is performed to make the AT control signal At zero, the light spot S.sub.2 on the optical card 1 is located just at the central position between the tracking tracks 4-2 and 4-3.
When recording is performed on an optical card at still higher density, the track pitch must be further decreased. In order to obtain a proper AT control signal upon recording/reproduction of information on/from a plurality of types of recording media having different track pitches, an optical system of a recording/reproduction apparatus must be re-designed or re-adjusted in a conventional system. However, it is not efficient to use a plurality of apparatuses having different optical systems, and it is troublesome and inefficient to adjust an optical system each time a different medium is used.